The Applicant has previously developed safety systems for industrial presses utilizing one or more light sources, each emitting a single continuous planar laser beam having a generally constant lateral width, and a light receiver for each beam. A single beam safety system is described in Australian patent number 738619, and a multiple beam system is described in Australian patent application number 2003215410, both of which are incorporated herein by reference. In such systems, both the light source(s) and the light receiver(s) are mounted on the moveable section of the industrial press. In the case of a press brake, the light source(s) and light receivers are mounted for movement with the moveable tool, and the tool (and the laser and light receivers mounted thereon) move relative to a stationary anvil or die section supporting a piece of sheet material to be bent by the tool. An example of such a safety system is described in International Patent Application No. PCT/AU00/00420, details of which are incorporated herein by reference. Especially when bending sheet materials using press brakes, the accuracy of the bending angle which is obtained depends, firstly, on the characteristics of the sheet, such as its thickness, which may vary from one sheet to another, and its physical and geometrical characteristics, on which the elastic return of the sheets depends. Furthermore, characteristics of the press itself, especially tolerance in the construction of the machine and tooling and wear on the tools, and also defamations appearing in the press at the instance of bending can also play a part in the bending angle.
Arrangements have previously been proposed to permit instantaneous automatic control of the bending operation in respect of the piece of material to be bent, so as to obtain, automatically and exactly, the desired bending angle of the material, while avoiding any manual intervention before and during the bending. Such automatic systems alleviate the need for manual adjustment and/or observation.
One such prior art automatic system is disclosed in U.S. Pat. No. 4,564,765, which proposes an opto-electronic measuring method and an apparatus for measuring material defamations in processing machines, particularly of the bending angle in free-bending die presses. This arrangement proposed one or two light spots produced on the deforming surface of the material, such as the bending side of a sheet metal plate, using a laser, infra-red emitting diode or the like, directed at an acute angle to the underside of the sheet material to be bent. The distance covered by the light spot, and the distance of the light spots from one another, respectively, are received by means of a photo detector, e.g., a diode line camera, which is set up at a distance and approximately perpendicular to the light spot or spots, and the measured values are compared with the index value in a microprocessor, e.g., by means of the control of the emotion depth of the bending die. At least one light source is arranged to strike the underside of the sheet metal plate at an angle relative to the bending axis. If the sheet metal plate is bent, the light spot appears to travel along the underside of the material. A sensor detects the extent of travel of the light spot and the resulting bending angle of the material can be determined form the displacement of the light spot relative to its starting position. However, such a system is limited in application to the bending of sheet material, and the accuracy of the determined angle of bend decreases with increasing bend angle due to the apparent travel of the light spot decreasing as the material is bent.
An alternative system has previously been proposed in U.S. Pat. No. 4,772,801. This patent discloses a light beam device for automatically controlling the bending operation when bending the press brake. The device includes an emitter mounted on one side of the press adapted to produce a large diameter light beam directed parallel to the bending axis of the work piece to be bent, i.e., along the length of the work piece to be bent, and the receiver including a box including a screen drilled with a plurality of holes arranged to form a plurality of light beams of small diameters. The box comprises a deflecting means intended to deflect the light beams onto a system of photo diodes arranged on the axis of rotation of a rotating motor and angle encoder. A micro coordinator is connected to the photo diodes and to the angle encoder, and permits the determination of the instantaneous bending angle of the work piece, and also makes possible determination of the speed of descent of the tool. However, aforesaid arrangement has limited application in needing to be attached to the lower table of a press and requires relatively complex moving parts and having an prism mounted for rotation about the axis of the image analyser. Furthermore, the accuracy of flexibility of measurement of the bend angle of the material is limited by the number of holes, and the separation distance between adjacent holes in a radial direction of the encoder from the axis of rotation, such that error in the determination of the bend angle can be significant.
An alternative system is disclosed in U.S. Pat. No. 4,550,586. This patent discloses a device employing a number of detectors mounted at a level of the upper surface of the die of an industrial press. The purpose of the detectors is to signal to a numerical control the moment a portion of a sheet of material to be bent begins to rise from the upper surface of the die, i.e., during bending. The device allows automatic determination of the thickness of the sheet and the area where the bend is to be located during the course of forming a bend. The thickness of the sheet can then be used by the numerical control system to correct the travel of the tool, without interrupting the forming cycle, in such a way as to obtain, with high precision, the required bend angle of the sheet material. However, with the detectors lying level with and adjacent to the upper surface of the die, such detectors are prone to damage or being accidentally obscured during bending operations and impeding and slowing down the loading and unloading of work pieces eg sheets of metal.
Many industrial machines, including press brakes, utilise a safety light beam system that will stop the machine operating if the safety beam is interrupted prior to a mute point, but will disable this stopping function after the mute point. Utilising machine operating speed up to the mute point is therefore important for productivity, whilst still needing to be able to stop the machine within sufficient travel distance to prevent injury to an operator. Thus, many machines are operated at high speed up to the mute point in order to maximise efficiency and productivity, and at a slower operating speed after the mute point.
For many safety systems that utilise either a single or multiple beams, operation is much the same. Interruption of one or more beams stops the machine. However, for broad or multi beam applications ie light beams having a broad 2 dimensional cross section (depth and height) in the order of centimeters or multiple beams effectively providing a broad safety zone, the tool tip may be several millimeters or centimeters away from the mute point when the first portion of the beam ahead of the tool tip reaches the mute point. In such scenarios, the machine would switch to slow speed because the broad beam has reached the mute point. This may lead to lost efficiency or productivity.
It would therefore be beneficial to provide a light beam safety system for an industrial machine that could recognise that part of the light beam had reached or passed the mute point and determine the required stopping distance.
With the aforementioned in view, it is one object of the present invention to provide a device for determining the progress of working of a selected piece of material whilst alleviating one or more of the problems of the aforementioned prior art.
It is a subordinate object of the present invention to provide an imaging device for determining an angle of bend of sheet material in an industrial press which allows constant determination of the angle of bend.
A further object of the present invention is to provide a safety beam system for a press brake which provides an adaptable broad beam protection zone to help maintain efficiency and productivity.